Change f to f1 in SplitFunction docs

[DanielVandH]

https://github.com/SciML/DiffEqDocs.jl/issues/688

[codecov[bot]]

Codecov Report

Merging #492 (0c90514) into master (03cdaed) will decrease coverage by 3.82%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #492      +/-   ##
==========================================
- Coverage   57.27%   53.45%   -3.82%     
==========================================
  Files          50       50              
  Lines        3703     3700       -3     
==========================================
- Hits         2121     1978     -143     
- Misses       1582     1722     +140     

Files Changed	Coverage Δ
src/scimlfunctions.jl	60.57% <ø> (-0.58%)	:arrow_down:

... and 10 files with indirect coverage changes

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