Update 07.ptx typo (#885)

Changing variable to x in the description to match the form given.

Author: jbunn3

source/precalculus/source/05-EL/07.ptx

@@ -127,7 +127,7 @@
       <activity xml:id="exponential-application-3">
         <introduction>
           <p>
-            In <m>2006</m>, <m>80</m> deer were introduced into a wildlife refuge. By <m>2012</m>, the population had grown to <m>180</m> deer. The population was growing exponentially. Recall that the general form of an exponential equation is <m>f(x)=a \cdot b^x</m>, where <m>a</m> is the initial value, <m>b</m> is the growth/decay factor, and <m>t</m> is time. We want to write a function <m>N(t)</m> to represent the deer population after <m>t</m> years.
+            In <m>2006</m>, <m>80</m> deer were introduced into a wildlife refuge. By <m>2012</m>, the population had grown to <m>180</m> deer. The population was growing exponentially. Recall that the general form of an exponential equation is <m>f(x)=a \cdot b^x</m>, where <m>a</m> is the initial value, <m>b</m> is the growth/decay factor, and <m>x</m> is time. We want to write a function <m>N(t)</m> to represent the deer population after <m>t</m> years.
           </p>
         </introduction>
         <task>