Fix pattern categories
Some patterns have a small defect in the yaml frontmatter (README.md). Categories key should be renamed to category so that everything shows up correctly on the website.

Acceptance criteria

- The pattern yaml frontmatters have been checked to conform with https://github.com/iluwatar/java-design-patterns/wiki/01.-How-to-contribute

identity-map/README.md intercepting-filter/README.md interpreter/README.md iterator/README.md
identity-map/README.md intercepting-filter/README.md interpreter/README.md iterator/README.md
diff --git a/identity-map/README.md b/identity-map/README.md
index a5bb4d38695a..6594f54eda9e 100644
--- a/identity-map/README.md
+++ b/identity-map/README.md
@@ -1,6 +1,6 @@
 ---
 title: Identity Map
-categories: Behavioral
+category: Behavioral
 language: en
 tags:
 - Performance
diff --git a/intercepting-filter/README.md b/intercepting-filter/README.md
index f5aef5d03032..aef71f4f5a76 100644
--- a/intercepting-filter/README.md
+++ b/intercepting-filter/README.md
@@ -1,6 +1,6 @@
 ---
 title: Intercepting Filter
-categories: Behavioral
+category: Behavioral
 language: en
 tags:
  - Decoupling
diff --git a/interpreter/README.md b/interpreter/README.md
index 61bf7501abbc..99d3c30e50ed 100644
--- a/interpreter/README.md
+++ b/interpreter/README.md
@@ -1,6 +1,6 @@
 ---
 title: Interpreter
-categories: Behavioral
+category: Behavioral
 language: en
 tags:
  - Gang of Four
diff --git a/iterator/README.md b/iterator/README.md
index 3f36d0070167..b49151eab125 100644
--- a/iterator/README.md
+++ b/iterator/README.md
@@ -1,6 +1,6 @@
 ---
 title: Iterator
-categories: Behavioral
+category: Behavioral
 language: en
 tags:
  - Gang of Four
