grammar nit, use a comma here (#4066)

diff --git a/docs/hazmat/primitives/symmetric-encryption.rst b/docs/hazmat/primitives/symmetric-encryption.rst
index aa577f0..126a918 100644
--- a/docs/hazmat/primitives/symmetric-encryption.rst
+++ b/docs/hazmat/primitives/symmetric-encryption.rst
@@ -136,7 +136,7 @@
             concatenated with a block counter (encoded as a 32-bit
             little-endian). If you have a separate nonce and block counter
             you will need to concatenate it yourself before passing it. For
-            example if you have an initial block counter of 2 and a 96-bit
+            example, if you have an initial block counter of 2 and a 96-bit
             nonce the concatenated nonce would be
             ``struct.pack("<i", 2) + nonce``.