Fix several misspellings of "Mixpanel"

Author: doppio

docs/mixpanel_flutter/Mixpanel/alias.html

@@ -64,8 +64,8 @@ <h1><span class="kind-method">alias</span> method
 <section class="desc markdown">
   <p>The alias method creates an alias which Mixpanel will use to remap one id to another.
 Multiple aliases can point to the same identifier.</p>
-<p><code>mixpane.alias("New ID", mixpane.distinctId)</code>
- <code>mixpane.alias("Newer ID", mixpane.distinctId)</code></p>
+<p><code>mixpanel.alias("New ID", mixpanel.distinctId)</code>
+ <code>mixpanel.alias("Newer ID", mixpanel.distinctId)</code></p>
 <p>This call does not identify the user after. You must still call both identify() and
 People.identify() if you wish the new alias to be used for Events and People.</p>
 <ul>

lib/mixpanel_flutter.dart

@@ -139,8 +139,8 @@ class Mixpanel {
   /// The alias method creates an alias which Mixpanel will use to remap one id to another.
   /// Multiple aliases can point to the same identifier.
   ///
-  ///  `mixpane.alias("New ID", mixpane.distinctId)`
-  ///  `mixpane.alias("Newer ID", mixpane.distinctId)`
+  ///  `mixpanel.alias("New ID", mixpanel.distinctId)`
+  ///  `mixpanel.alias("Newer ID", mixpanel.distinctId)`
   ///
   /// This call does not identify the user after. You must still call both identify() and
   /// People.identify() if you wish the new alias to be used for Events and People.