[1.7.x] Various documentation typo/spelling fixes

Errors detected by Topy (https://github.com/intgr/topy), all changes
verified by hand.

Backport of 11d453bcad from master

docs/topics/forms/index.txt

@@ -96,7 +96,7 @@ The standard pattern for processing a form in a view looks like this:
 
    def contact(request):
        if request.method == 'POST': # If the form has been submitted...
-           # ContactForm was defined in the the previous section
+           # ContactForm was defined in the previous section
            form = ContactForm(request.POST) # A form bound to the POST data
            if form.is_valid(): # All validation rules pass
                # Process the data in form.cleaned_data

docs/topics/i18n/translation.txt

@@ -124,7 +124,7 @@ specified with Python's standard named-string interpolation syntax. Example::
 This technique lets language-specific translations reorder the placeholder
 text. For example, an English translation may be ``"Today is November 26."``,
 while a Spanish translation may be ``"Hoy es 26 de Noviembre."`` -- with the
-the month and the day placeholders swapped.
+month and the day placeholders swapped.
 
 For this reason, you should use named-string interpolation (e.g., ``%(day)s``)
 instead of positional interpolation (e.g., ``%s`` or ``%d``) whenever you

docs/topics/testing/advanced.txt

@@ -51,7 +51,7 @@ The following is a simple unit test using the request factory::
             # Create an instance of a GET request.
             request = self.factory.get('/customer/details')
 
-            # Recall that middleware are not suported. You can simulate a
+            # Recall that middleware are not supported. You can simulate a
             # logged-in user by setting request.user manually.
             request.user = self.user