DOC: adapt example of groupby filter and transform

[stijnvanhoey]

In this pull request the filter and transform examples are converted to a running doctest

[codecov-io]

Codecov Report

Merging #15624 into master will decrease coverage by -0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #15624      +/-   ##
==========================================
- Coverage   91.01%      91%   -0.02%     
==========================================
  Files         143      143              
  Lines       49301    49301              
==========================================
- Hits        44873    44864       -9     
- Misses       4428     4437       +9

Impacted Files	Coverage Δ
pandas/core/groupby.py	94.98% <ø> (ø)	✅
pandas/io/gbq.py	25% <0%> (-58.34%)	❌
pandas/tools/merge.py	91.76% <0%> (-0.35%)	❌
pandas/core/frame.py	97.87% <0%> (-0.1%)	❌
pandas/tseries/index.py	95.39% <0%> (+0.09%)	✅
pandas/core/common.py	91.3% <0%> (+0.33%)	✅

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 54e71a7...ee90944. Read the comment docs.

[jreback]

thanks!