stylistic issues in a couple of cmd docs

R=rsc
http://go/go-review/1017028

src/cmd/nm/doc.go

@@ -4,7 +4,7 @@
 
 /*
 
-nm is a version of the Plan 9 nm command documented at
+This nm is a version of the Plan 9 nm command documented at
 
 	http://plan9.bell-labs.com/magic/man2html/1/nm
 

src/cmd/prof/doc.go

@@ -4,7 +4,7 @@
 
 /*
 
-prof is a rudimentary real-time profiler.
+Prof is a rudimentary real-time profiler.
 
 Given a command to run or the process id (pid) of a command already
 running, it samples the program's state at regular intervals and reports