DOC: Disambiguate 'where' in boolean indexing-10min.rst (#12661)
 - [x] closes #12661

Per @jorisvandenbossche comment in this thread (https://github.com/pandas-dev/pandas/pull/12671), remove code markdown from 'where' to not confuse the example with the `where()` method.

## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14708?src=pr) is 85.20% (diff: 100%)
> Merging [#14708](https://codecov.io/gh/pandas-dev/pandas/pull/14708?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

```diff
@@             master     #14708   diff @@
==========================================
  Files           143        143          
  Lines         50796      50787     -9   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          43285      43274    -11   
- Misses         7511       7513     +2   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [880de30...1093b0a](https://codecov.io/gh/pandas-dev/pandas/compare/880de3054c7ce3a6c48e295837207787dc73350f...1093b0a8ba65c0b1931409c7d6843080afe0242b?src=pr) Thanks for the feedback @jreback. Expanded a bit on the example.   @mroeschke Thanks!