docs: add index using relative link

sorting/README.md

@@ -1,11 +1,12 @@
 # Sorting algorithms
 
-### c-or-cpp
+### C or C++
 
 1. [Bubble Sort](c-or-cpp/bubble-sort.cpp)
 2. [Insertion Sort](c-or-cpp/insertion-sort.cpp)
+3. [Selection Sort](c-or-cpp/selection-sort.cpp)
 
-### python
+### Python
 
 1. [Bubble Sort](python/bubble-sort.py)
 

strings/README.md

@@ -1,5 +1,5 @@
 # String operations
 
-### c-or-cpp
+### C or C++
 
 1. [Palindrome Check](c-or-cpp/palindrome.c)