fixed an nroff syntax mistake

docs/curl-config.1

@@ -26,7 +26,7 @@ Shows the complete set of libs and other linker options you will need in order
 to link your application with libcurl.
 .IP "--prefix"
 This is the prefix used when libcurl was installed. Libcurl is then installed
-in \$prefix/lib and its header files are installed in \$prefix/include and so
+in $prefix/lib and its header files are installed in $prefix/include and so
 on. The prefix is set with "configure --prefix".
 .IP "--version"
 Outputs version information about the installed libcurl.