git-gui: fix typo in lib/spellcheck.tcl

Signed-off-by: Michele Ballabio <barra_cuda@katamail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
diff --git a/lib/spellcheck.tcl b/lib/spellcheck.tcl
index 76a2f17..9be7486 100644
--- a/lib/spellcheck.tcl
+++ b/lib/spellcheck.tcl
@@ -70,7 +70,7 @@
 					[mc "Reverting dictionary to %s." $s_lang]]
 			}
 		} else {
-			error_popup [mc "Spell checker sliently failed on startup"]
+			error_popup [mc "Spell checker silently failed on startup"]
 		}
 		return
 	}