cmd/gc: fix -m description in help output

<pre>What steps will reproduce the problem?
1. 8g -h

What is the expected output?
...
-m print about escape analysis and inlining
...

What do you see instead?
...
-m print about moves to heap
...

Which revision are you using?  (hg identify)
8g version weekly.2012-01-27 11507</pre>
