Update custom pipes title to impure pipes

src/data/roadmaps/angular/content/custom-pipes@BOYXGfULJRiP-XOo_lNX3.md

@@ -1,4 +1,4 @@
-# Custom Pipes
+# Impure Pipes
 
 Pipes to transform strings, currency amounts, dates, and other data for display. Pipes are simple functions in template expressions to accept an input value and return a transformed value. Pipes are helpful because you can use them throughout your application while only declaring each pipe once. For example, you would use a pipe to show the date as April 15, 1988, rather than the raw string format.