extra {} in doc fixes davidcarlisle/dpctex#45

required/tools/bm.dtx

@@ -178,7 +178,7 @@
 % spacing). So the third example produces identical output to the first
 % (but \TeX\ takes more time producing it).
 %
-% In the fourth example the |\mathrm{\bm{g}}| is essentially
+% In the fourth example the |\mathrm{\bm{{g}}}| is essentially
 % equivalent to |\mathrm{\mbox{\boldmath$g$}}|. Currently math alphabet
 % settings are not passed down to `nested' math lists, and so in this
 % example, the |\mathrm| has no effect, and a bold math italic $\bm g$