fix(paginator): change class name of increment and decrement icons

Fix such that the previous page navigation button contains an icon with class decrement,
and the next page navigation button contains an icon with class increment. Also updated
the scss variables with similarly wrong names.
Previously they were incorrectly swapped.

Noticed during fix for #9278

Author: lharries

src/lib/paginator/paginator.scss

@@ -15,8 +15,8 @@ $mat-paginator-button-margin: 8px;
 $mat-paginator-button-icon-height: 8px;
 $mat-paginator-button-icon-width: 8px;
 
-$mat-paginator-button-decrement-icon-margin: 12px;
-$mat-paginator-button-increment-icon-margin: 16px;
+$mat-paginator-button-increment-icon-margin: 12px;
+$mat-paginator-button-decrement-icon-margin: 16px;
 
 .mat-paginator {
   display: block;
@@ -74,16 +74,16 @@ $mat-paginator-button-increment-icon-margin: 16px;
 }
 
 .mat-paginator-increment {
-  margin-left: $mat-paginator-button-decrement-icon-margin;
+  margin-left: $mat-paginator-button-increment-icon-margin;
   [dir='rtl'] & {
-    margin-right: $mat-paginator-button-decrement-icon-margin;
+    margin-right: $mat-paginator-button-increment-icon-margin;
   }
 }
 
 .mat-paginator-decrement {
-  margin-left: $mat-paginator-button-increment-icon-margin;
+  margin-left: $mat-paginator-button-decrement-icon-margin;
   [dir='rtl'] & {
-    margin-right: $mat-paginator-button-increment-icon-margin;
+    margin-right: $mat-paginator-button-decrement-icon-margin;
   }
 }