[fortran/en] declares single precision pi without extraneous digits

[Beliavsky]

Addresses https://github.com/adambard/learnxinyminutes-docs/issues/5170.

The previous version had a line

real, parameter :: PI = 3.1415926535897931

that was misleading, since real variables are single precision by default in Fortran and will not store all the digits shown.

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