docs(guide): update compiler guide with minor grammatical fixes

Minor changes to grammar. Changed sentence "But the declarative language
is also limited, since it does not allow you to teach the browser new syntax."
to now read "However, the declarative language is also limited, as it does not
allow you to teach the browser new syntax."
However is a less informal start to a sentence, and replacing "since"
correctly references extent/degree rather than comparison of time.

Author: biversen21

docs/content/guide/compiler.ngdoc

@@ -25,8 +25,8 @@ browser how the window size needs to be divided in half so that the center is fo
 center needs to be aligned with the text's center. Simply add an `align="center"` attribute to any
 element to achieve the desired behavior. Such is the power of declarative language.
 
-But the declarative language is also limited, since it does not allow you to teach the browser new
-syntax. For example there is no easy way to get the browser to align the text at 1/3 the position
+However, the declarative language is also limited, as it does not allow you to teach the browser new
+syntax. For example, there is no easy way to get the browser to align the text at 1/3 the position
 instead of 1/2. What is needed is a way to teach the browser new HTML syntax.
 
 Angular comes pre-bundled with common directives which are useful for building any app. We also