Change this in Pages Preferences.
The Pages'09 User Manual says this on page 117:
Automatically Substituting Text
Pages can be set to recognize and replace text you don’t want with text you do want. For example, when you type “teh,” Pages can automatically change it to “the.”
To set up automatic text substitution:
- 1 Choose Pages > Preferences.
- 2 Click Auto-Correction and make settings as desired.
Use smart quotes: Automatically convert single and double quotation marks to smart (“curly”) quotes so that opening and closing quotation marks are not identical.
Fix capitalization: The first word in a sentence begins with a uppercase letter.
Superscript numerical suffixes: Automatically convert the letters in “1st,” “2nd,” “3rd,”
and so on into superscripts.
Automatically detect email and web addresses: Set Pages to automatically detect that something you’ve typed is an email address or a URL. Email and web addresses you type automatically become hyperlinks for Mail or Safari.
Automatically detect lists: Use automatic list generation.
Automatically use spell checker suggestions: Automatically replace misspelled words
if there is only one option in the spelling dictionary.
Symbol and text substitution: Replace one or more characters with one or more different characters, and then use the rows in the table to define and activate specific substitutions. For example, when you type (c), you can have Pages automatically convert it to © by putting a checkmark in the On column. To add a row to the table to define your own substitution, click the Add button. To remove a selected item, click the Delete (–) button.
After you specify substitution settings, they’ll apply to any text you change or add in any Pages document.
You can download the User Guide from the Pages Help Menu. Well worth investing some time in reading at least the first 3 chapters.
Hope this helps.