src/doc/trpl -> src/doc/book

The book was located under 'src/doc/trpl' because originally, it was
going to be hosted under that URL. Late in the game, before 1.0, we
decided that /book was a better one, so we changed the output, but
not the input. This causes confusion for no good reason. So we'll change
the source directory to look like the output directory, like for every
other thing in src/doc.

mk/docs.mk

@@ -65,7 +65,7 @@ ERR_IDX_GEN = $(RPATH_VAR2_T_$(CFG_BUILD)_H_$(CFG_BUILD)) $(ERR_IDX_GEN_EXE)
 
 D := $(S)src/doc
 
-DOC_TARGETS := trpl nomicon style error-index
+DOC_TARGETS := book nomicon style error-index
 COMPILER_DOC_TARGETS :=
 DOC_L10N_TARGETS :=
 
@@ -194,12 +194,12 @@ docs: $(DOC_TARGETS)
 doc: docs
 compiler-docs: $(COMPILER_DOC_TARGETS)
 
-trpl: doc/book/index.html
+book: doc/book/index.html
 
-doc/book/index.html: $(RUSTBOOK_EXE) $(wildcard $(S)/src/doc/trpl/*.md) | doc/
+doc/book/index.html: $(RUSTBOOK_EXE) $(wildcard $(S)/src/doc/book/*.md) | doc/
 	@$(call E, rustbook: $@)
 	$(Q)rm -rf doc/book
-	$(Q)$(RUSTBOOK) build $(S)src/doc/trpl doc/book
+	$(Q)$(RUSTBOOK) build $(S)src/doc/book doc/book
 
 nomicon: doc/nomicon/index.html