Singular extension search tweak

Tweak depth.

STC https://tests.stockfishchess.org/tests/view/5f2d22ec61e3b6af64881f40
LLR: 2.94 (-2.94,2.94) {-0.50,1.50}
Total: 17984 W: 2603 L: 2441 D: 12940
Ptnml(0-2): 133, 1751, 5094, 1849, 165

LTC https://tests.stockfishchess.org/tests/view/5f2d5a6a61e3b6af64881f7f
LLR: 2.95 (-2.94,2.94) {0.25,1.75}
Total: 85808 W: 5956 L: 5621 D: 74231
Ptnml(0-2): 149, 4748, 32785, 5063, 159

closes https://github.com/official-stockfish/Stockfish/pull/2950

fixes two README.md typos:
fixes https://github.com/official-stockfish/Stockfish/issues/2932

bench: 4022669

README.md

@@ -58,7 +58,7 @@ Currently, Stockfish has the following UCI options:
 
   * #### Use NNUE
     Toggle between the NNUE and classical evaluation functions. If set to "true",
-    the network parameters must be availabe to load from file (see also EvalFile).
+    the network parameters must be available to load from file (see also EvalFile).
 
   * #### EvalFile
     The name of the file of the NNUE evaluation parameters. Depending on the GUI the
@@ -138,7 +138,7 @@ Currently, Stockfish has the following UCI options:
   * #### Debug Log File
     Write all communication to and from the engine into a text file.
 
-## classical and NNUE evaluation
+## Classical and NNUE evaluation
 
 Both approaches assign a value to a position that is used in alpha-beta (PVS) search
 to find the best move. The classical evaluation computes this value as a function

src/search.cpp

@@ -1072,7 +1072,7 @@ moves_loop: // When in check, search starts from here
       // then that move is singular and should be extended. To verify this we do
       // a reduced search on all the other moves but the ttMove and if the
       // result is lower than ttValue minus a margin, then we will extend the ttMove.
-      if (    depth >= 6
+      if (    depth >= 7
           &&  move == ttMove
           && !rootNode
           && !excludedMove // Avoid recursive singular search