Minor updates to the "Additive Bias, Multiplicative Bias and Percent Bias" tutorial

[Steph-Chong]

@tennlee @nicholasloveday @durgals

This PR makes some minor updates to the bias tutorial

• Adds an "Introduction" header (I think this helps with readthedocs rendering)
• Changes the header levels for "Additive bias", "Multiplicative bias" and "Percent bias" from third level to second level headers (doing so resolves some readthedocs rendering issues)
• Changes spelling of "standardized" to "standardised"
• Adds backticks to one instance of scores (in the "What to try next?" section)
• Changed three instances of "additive bias precentage" to "percent bias" in code comments (@durgals - did I make these changes correctly?)

Some additional questions I had:

1. Should the first level header at the start of the tutorial now be updated, perhaps to something along the lines of "Additive Bias, Multiplicative Bias and Percent Bias" or "Additive, Multiplicative and Percent Bias"?

2. Should the title as displayed in the tutorial gallery and the readthedocs left hand side bar be similarly changed?

3. Should a references section be added at the end of this tutorial? (I realise some of the tutorials have a references section and others do not.)

[durgals]

@tennlee @nicholasloveday @durgals

• Changed three instances of "additive bias precentage" to "percent bias" in code comments (@durgals - did I make these changes correctly?)

@Steph-Chong : Yes.

[Steph-Chong]

Some additional questions I had:

1. Should the first level header at the start of the tutorial now be updated, perhaps to something along the lines of "Additive Bias, Multiplicative Bias and Percent Bias" or "Additive, Multiplicative and Percent Bias"?

Following offline discussion with @tennlee, I updated the first level header (title) in the title to "Additive Bias, Multiplicative Bias and Percent Bias".

[Steph-Chong]

Some additional questions I had:

2. Should the title as displayed in the tutorial gallery and the readthedocs left hand side bar be similarly changed?

3. Should a references section be added at the end of this tutorial? (I realise some of the tutorials have a references section and others do not.)

2. Tennessee updated the title in the tutorial gallery (and consequently, in the left hand side bar on readthedocs), and pushed it to this PR

3. Following offline discussion with Tennessee, I added a references section (I used the references from the Percent Bias docstring and the references currently in included.md for Additive Bias and Multiplicative Bias).