Fix README.md grammar "an attacker"

English nouns begin with an indefinite article "a" or "an". Usually "a"
is used when the word that follows starts with a consontant, and "an"
is used when the word that follows starts with a vowel.

The word "attacker" starts with a vowel and has no different
pronunciation of the first letter, so the correct indefinite article to
use is "an". The correct way to pronounce it is "an attacker".

This fixes an error introduced in commit db66648.

1 files changed, 1 insertions, 1 deletions

diff --git a/README.md b/README.md
index 9d29632..376fb66 100644
--- a/README.md
+++ b/README.md
@@ -37,7 +37,7 @@ however, client-side configuration for max size, the `data-max-size` attribute
 on the file input in `pages/upload_form.swig`.
 
 Make sure to disable PHP from being executed on the file download
-domain/directory (e.g a.site.com), otherwise a attacker can upload a malicious
+domain/directory (e.g a.site.com), otherwise an attacker can upload a malicious
 .php file and execute it on your server.
 
 ### Apache