rustc.1: fix typo

Uses the same wording as ``config.rs`` now.

Author: Philipp Gesang

man/rustc.1

@@ -68,7 +68,7 @@ AST nodes and blocks with IDs), or flowgraph=<nodeid> (graphviz
 formatted flowgraph for node)
 .TP
 \fB\-\-dep-info\fR [FILENAME]
-Output dependency info to <filename> after compiling, in o format suitable
+Output dependency info to <filename> after compiling, in a format suitable
 for use by Makefiles.
 .TP
 \fB\-\-sysroot\fR PATH