man: fix grammar

an user -> a user

Signed-off-by: Abdur Rehman <arehmanq199@gmail.com>
Reviewed-by: Yong Bakos <ybakos@humanoriented.com>
diff --git a/man/weston.ini.man b/man/weston.ini.man
index 2eac098..5466848 100644
--- a/man/weston.ini.man
+++ b/man/weston.ini.man
@@ -360,7 +360,7 @@
 .RE
 .RS
 .PP
-Optionally, an user may specify a modeline, such as:
+Optionally, a user may specify a modeline, such as:
 .PP
 .nf
 .in +4n