Fix typo in DistributedArchitectureGuide (elastic#128373)

Fixes two typos in the `DistributedArchitectureGuide` where `node` was
incorrectly called `NodeB`.

Author: joshua-adams-1

docs/internal/DistributedArchitectureGuide.md

@@ -46,7 +46,7 @@ to communicate with Elasticsearch.
 
 (Sketch of important classes? Might inform more sections to add for details.)
 
-(A NodeB can coordinate a search across several other nodes, when NodeB itself does not have the data, and then return a result to the caller. Explain this coordinating role)
+(A node can coordinate a search across several other nodes, when the node itself does not have the data, and then return a result to the caller. Explain this coordinating role)
 
 ### Node Roles