revise: signal-cell -> single-cell (#56)

Author: zafstojano

docs/06-advanced-topics/cancer-primer.md

@@ -31,7 +31,7 @@ These two events are analogous to increased expression of [**oncogenes**](https
 
 ## The Evolutionary Basis for Controlled Growth
 
-For bacteria and other single-cell organisms, the fastest growing cell claims the most territory, has the most progeny, and wins the evolutionary battle. Growth is only limited by resources and environmental conditions like temperature. In multicellular organisms, resources are shared and the cells must function as a unit if the whole is to survive. To achieve that end, multicellular organisms have evolved elaborate control mechanisms to manage growth and keep individual cells in check. These additional mechanisms are maintained, through systems of genes working together, or **pathways**. In cancerous cells, these pathways are disabled by the loss of functional tumor suppressors, returning the cell to a state like the primitive signal-cell organism. The takeaway is that cells are wired for growth naturally, which is why removing controls are so consequential.
+For bacteria and other single-cell organisms, the fastest growing cell claims the most territory, has the most progeny, and wins the evolutionary battle. Growth is only limited by resources and environmental conditions like temperature. In multicellular organisms, resources are shared and the cells must function as a unit if the whole is to survive. To achieve that end, multicellular organisms have evolved elaborate control mechanisms to manage growth and keep individual cells in check. These additional mechanisms are maintained, through systems of genes working together, or **pathways**. In cancerous cells, these pathways are disabled by the loss of functional tumor suppressors, returning the cell to a state like the primitive single-cell organism. The takeaway is that cells are wired for growth naturally, which is why removing controls are so consequential.
 
 ## Control Signals and the Immune System